a) Field of the Invention
The present invention relates to multi-layer blow molded articles of a polypropylene which are excellent in surface gloss, transparency, impact resistance and moldability.
b) Description of the Related Art
In general, polypropylenes are characteristically inexpensive and excellent in mechanical properties, hygienic adaptability and resistance to water vapor-permeation, and their molded articles have a good appearance. Therefore, they are widely used for various packaging materials and blow molded articles. Although commercially available isotactic polypropylenes show an excellent moldability upon forming blow molded articles, however, the surface of the molded articles is poor in gloss and hence lacks an impression of high grade; the molded articles have an insufficient transparency with a whiteness involved so that the appearance of their contents seems to differ in color from that of the actual matter; and when a liquid material with a high specific gravity such as an aqueous solution is fed as a product in a relatively large container with an internal volume of 100 ml or more, the container is occasionally broken due to the poor drop impact strength of the molded article particularly at low temperatures as in cold zones and refrigerators. The blow molded articles give rise to such problems and therefore their improvement has been desired.
In order to improve the surface gloss, it has been proposed to use a crystalline isotactic ethylene-propylene random copolymer having a specific ethylene content, melt flow index and molecular weight distribution in the outermost layer of a multi-layer plastic container (Japanese Patent Laid-Open No. 215529/1990). According to this method, however, the gloss is still insufficient to give an impression of high grade, though it is improved to some extent, and the transparency and the impact resistance are not improved. It has also been proposed to use a syndiotactic polypropylene or a resin composition comprising a syndiotactic polypropylene and an isotactic polypropylene in the outer layer (Japanese Patent Laid-Open No. 032556/1995). According to this method, the surface gloss is well improved and the transparency as a total laminate can also be excellent because the transparency of the outer layer is superior to that of the inner layer. However, substantially no improvement is seen in the drop impact strength of the molded article particularly at low temperatures.
In order to improve the drop impact resistance of a blow molded article particularly at low temperatures, it can be thought to impart impact resistance to the resin composition used in the outermost layer in a multi-layer blow molded article or to that used in the layer itself in a single-layer blow molded article. As a general method for imparting impact resistance, a variety of elastomers are incorporated. However, if an elastomer such as EPR is incorporated in an isotactic polypropylene, the impact resistance of the resultant blow molded article at low temperatures is significantly improved, but the surface gloss thereof is still inferior to that of the blow molded article made from a syndiotactic polypropylene, and the transparency is often degraded. Accordingly, desired blow molded articles which are excellent in surface gloss and superb in transparency have never been obtained. Thus, it has been very difficult to obtain a blow molded article of a polypropylene excellent in all of the surface gloss, transparency, drop impact resistance particularly at low temperatures, and moldability.